A refining process of mechanically stirring molten iron with an impeller has heretofore been applied mainly to desulfurization of blast furnace-derived molten pig-iron (for example, Patent References 1 to 4). In that regard, for enhancing the stirring efficiency, proposed is a stirring method where the rotation axis of the impeller is kept decentered from the central axis of the refining vessel (Patent Reference 3). According to the method, it is said that the revolutions per minute (r.P.m) could be reduced in a case of obtaining a predetermined desulfurization efficiency, and the life of the impeller could be prolonged.
On the other hand, in the production of molten stainless steel, a process of obtaining molten pig-iron or molten steel by the use of an electric furnace is the mainstream. In that case, CaF2 (fluorite) may be incorporated in the slag in the electric furnace, or CaF2 may be incorporated in the slag during the decarburizing stage, whereby the desulfurization can be attained relatively efficiently, and accordingly, a step of mechanically stirring molten pig-iron or molten steel is not specifically needed.
Recently, however, in use of steel slag as a ground or roadbed material, the content of the fluorine ingredient therein has become restricted, and therefore use of CaF2-free slag has increased. In that case, the desulfurization capability of slag lowers, and therefore, in case where an ultra-low S molten stainless steel having an S content of, for example, at most 0.005% by mass is produced, it has become necessary to apply separate desulfurization treatment to the electric furnace molten pig-iron or steel for the purpose of reducing the desulfurization load in the already-existing steel-making process
Regarding the desulfurization treatment, it has been confirmed that the same mechanical stirring method as that for blast furnace molten pig-iron is effective also for chrome-containing molten pig-iron or steel for stainless steel. For example, in case where CaO is used as the desulfurizing agent and when chrome-containing molten pig-iron or steel is mechanically stirred along with the desulfurizing agent (CaO-based slag), then the desulfurization reaction of the following formula (1) goes on. The generated oxygen reacts with the deoxidizing ingredient (for example, Si) in the molten iron, according to the following formula (2):(CaO)+[S]=(CaS)+[O]  (1)[Si]+2[O]=(SiO2)  (2)